An alloyed plated steel sheet such as an alloyed-Zn-plated steel sheet is better than ordinary (Zn-)plated steel sheet in that continuous spot-welding can be easily carried out, the adhesion of electrophoretic coating film is good, the corrosion resistance is better, etc. and, therefore, sheet of this type is being widely used in the automobile industry and other fields.
Conventionally, alloyed-Zn-plated steel sheet is manufactured by heat-treating Zn-plated steel sheet prepared by hot-dip plating or electrolytic plating. However, hot-dip plating is not suitable for thin coating of less than 30 g/m.sup.2 and one side plating, and hot-dipplated steel sheet is inferior in uniformity in thickness of the plating layer in the londitudinal direction and the transverse direction. Electrolytic plating is not suitable for plating of thicker than 50 g/m.sup.2 because the plating cost rises steeply with the increase in coating weight. In contrast, the vacuum vapor deposition process is advantageous in that the coating weight can be relatively easily controlled and uniform thickness is easily achieved. Therefore, processes for preparing alloyed Zn-plated steel sheet utilizing vacuum vapor deposition have been proposed and being practiced (JP-A-61-195965 for instance). According to this process, cold-rolled steel coil is plated with Zn on one or both sides by vacuum vapor deposition and, thereafter, is heat-treated for alloying at 250.degree.-350.degree. C. for 1-15 hours in a non-oxidizing or weakly reducing atmosphere in a batch annealing furnace.
In the meanwhile, for manufacturing steel sheets for deep-drawing, aluminum(Al)-killed steel and Ti-killed steel are used. With Al-killed steel sheet, alloyed-Zn-plated steel sheet can be manufactured by vacuum vapor deposition and heat treatment in accordance with the above-described known process. With Ti-killed steel sheet, however, there is a problem that the formed alloy layer is inferior in powdering resistance and easily scales off.
For ordinary deep drawing purposes, Al-killed steel sheet suffices. Recently, however, more and more complicated shaping is being required. For such purposes, Ti-killed steel, which has better workability and formability, must be used. Therefore, alloyed-Zn-plated steel sheet made of Ti-killed steel sheet also must be provided with good powdering resistance.
We have conducted studies to determine the cause of the inferior powdering resistance of alloyed-Zn-plated steel sheet made of Ti-killed steel sheet focusing our attention on the structure of alloyed plating layer. As a result of our studies, we have found that in the case of Ti-killed steel, very brittle intermetallic compounds of iron(Fe) and Zn are formed when the Zn-plated steel sheet is heated to temperatures outside of a specific temperature range and that this specific temperature range differs depending upon the Ti content. That is, the temperature is in excess of 320.degree. C. when the Ti content is 0.05% and in excess of 260.degree. C. when the Ti content is 0.3%. Further, once a brittle intermetallic compound layer is formed, deterioration of powdering resistance is inevitable irrespective of the alloying conditions under which the plated steel sheet is treated. Thus we have found that the aforesaid problem can be solved by maintaining the substrate Ti-killed steel sheet at a low temperature set relative to the Ti content during the vacuum vapor deposition and heating the plated steel sheet in a non-oxidizing atmosphere, preferably in a batch heat treatment annealing furnace, at a temperature within the temperature range in which the Fe content of the formed alloyed layer is wisthin a prescribed range.